1. Field
The present invention relates to the field of fiber optics. In particular, the present invention constitutes a cross-polarization interference canceler (XPIC) and its corresponding method of operation that optimizes bandwidth efficiency over an optical fiber transmission medium and mitigates dispersion effects or any loss of optical field orthogonality incurred during propagation through the optical fiber.
2. Related Art
Due to increased demand for fast data transmissions, optical communication networks are being utilized more frequently. Normally, a fiber optic communication link includes a set of transceivers coupled together by optical fiber supporting one or more fiber optic transmission channels. Each transceiver includes a transmitter and receiver. The transmitter converts an electrical signal to a single optical signal, which is applied to a fiber optic transmission medium. The receiver converts the optical signal back to an electrical signal, which may be routed through electrical wire or processed by a computer for example.
Typically, the electric field of the received optical signal has time varying polarization, and the optical receiver must either be insensitive to the field polarization, or track the optical field polarization to recovery the transmitted information.
Referring to FIG. 1, a block diagram of a conventional coherent fiber optic communication link utilizing a polarization diversity receiver 100 is shown. This type of receiver is insensitive to the polarization of the received optical signal field. Receiver 100 includes a pair of polarization beam splitters 110 and 120. A first polarization beam splitter 110 receives an, incoming optical signal from the optical fiber. This incoming optical signal comprises an electromagnetic (EM) plane wave with an electric field 130 (referred to as xe2x80x9cxcex5S(t)xe2x80x9d) that is amplitude and/or phase modulated with information bearing in-phase (I(t)) and quadrature (Q(t)) waveforms.
For clarity sake, the horizontal ({circumflex over (x)}) and vertical (ŷ) directions are defined to coincide with the polarization axes of the polarization beam splitters at receiver 100. The polarization states of the transmitted electric field (referred to as xe2x80x9cxcex5T(t)xe2x80x9d) and received electric field (xcex5S(t)) 130 are assumed arbitrary with respect to the {circumflex over (x)} and ŷ directions.
Polarization beam splitter 110 separates received electric field xcex5S(t) 130 into a horizontal ({circumflex over (x)}) field component and a vertical (ŷ) field component. These {circumflex over (x)} and ŷ components of xcex5S(t) 130 are routed into double-balanced optical receivers (DBORs) 140 and 150. Polarization beam splitter 120 receives an optical signal from a local oscillator (LO) laser, in particular a non-modulated EM plane wave with an electric field (xcex5L(t)) 135. A xe2x80x9cDBORxe2x80x9d is a device that performs the function of an optical mixer, multiplying the optical inputs and removing the resulting high frequency components through the inherent bandlimiting of the photodiodes. This multiplication process is polarization sensitive, however, and only the component of xcex5S(t) 130 with the same polarization as xcex5L(t) 135 is detected.
The frequency of the LO laser is adjusted such that the difference between the optical LO frequency and the carrier frequency of xcex5S(t) is equal to the desired intermediate carrier frequency xcfx89IF of the DBOR output current. A demodulator 170 then demodulates the IF signal and recovers the baseband information bearing waveforms I(t) and Q(t).
More specifically, the {circumflex over (x)} component of xcex5S(t) is routed to DBOR 140 and the ŷ component is routed to DBOR 150. Each DBOR 140 or 150 includes, for example, a directional coupler and photodiodes connected in series. The {circumflex over (x)} component of xcex5L(t) is also routed to DBOR 140 and the ŷ component is routed to DBOR 150. If the polarization state of xcex5L(t) is adjusted to be linearly polarized at forty-five (45) degrees with respect to the {circumflex over (x)} axis, then DBOR 140 will respond with an electrical current (referred to as xe2x80x9ci1(t)xe2x80x9d) having a magnitude that is proportional to the {circumflex over (x)} component of xcex5S(t) 130, and DBOR 150 will respond with a current (referred to as xe2x80x9ci2(t)xe2x80x9d) having a magnitude that is proportional to the ŷ component of xcex5S(t) 130 and having the same phase as i1(t).
The output electrical currents of DBORs 140 and 150 are combined to produce a resultant electrical signal current 160 (referred to as xe2x80x9ci3(t)xe2x80x9d). Thus, since any polarization state can be resolved into {circumflex over (x)} and ŷ components, polarization diversity receiver 100 will respond to a received optical signal field with arbitrary polarization. Although polarization diversity receiver 100 is able to detect a received optical signal field with arbitrary polarization, it fails to take advantage of the potential to transmit independent optical signals across the optical medium in the same frequency band, but with orthogonal polarization states, and thereby increase the link bandwidth efficiency.
Under ideal conditions, two conventional polarization tracking receivers (not shown) could reconstruct two received optical signals having fields with orthogonal polarization states by tracking the polarization of each signal field. Theoretically, if the polarization of the optical LO in each receiver is adjusted to match the polarization of the electric field of one of the optical signals, then the other optical signal will be rejected. However, in practice, the orthogonality of the two optical signal fields would be lost to some extent during propagation through the optical fiber. Therefore, a system having two conventional polarization tracking receivers would incur a signal crosstalk penalty, also known as cross polarization interference (XPI). This would have adverse effects on the quality and reliability of the optical signaling.
Besides cross polarization interference, the optical signal may experience chromatic and/or polarization mode dispersion. In general, dispersion is problematic when a light pulse, normally associated with a particular period of time, begins to occupy portions of the time period associated with adjacent light pulses. A solution to overcome dispersion involves reducing the transmission length of the optical fiber. One way to reduce the transmission length is to place regenerative repeaters at selected intervals of the optical fiber. Regenerators require signal detection, electrical clock recovery circuitry, and means of generating a new optical signal from the recovered electrical signal. The use of repeaters, therefore, significantly increases the construction costs for an optical communication network.
It would therefore be desirable to develop a cross-polarization interference canceler (XPIC) and a method that optimizes bandwidth efficiency over the optical fiber by enabling two optical signals transmitted in the same frequency band but with orthogonal polarization to be recovered at the receiver. This could correct for dispersion effects or any loss of optical field orthogonality incurred during propagation through the optical fiber, and optimally reconstruct the information bearing modulation waveforms at the receiver.
As described herein, implemented in both coherent and non-coherent optical systems, exemplary embodiments of a receiving device including a cross polarization interference canceler (XPIC) are shown. For each of these embodiments, the XPIC optimizes bandwidth efficiency of an optical link by enabling the reconstruction of two optical signals transmitted with generally orthogonal polarization states and routed over a single fiber optic transmission medium in the same frequency band.